(1) Field of the Invention
The present invention relates to an optical pickup device having the function of detecting signals such as a playback/recording signal and various servo signals used in an optical head device serving as a main part of an optical information processor performing processing, such as recording, playback or erasure of information, on an optical information recording medium such as an optical disk.
(2) Background Art
To record and play back compact discs (CDs) having the largest share of the optical disk market, near-infrared semiconductor lasers in a wavelength band of 780 nm to 820 nm are currently used. On the other hand, to record and play back digital versatile discs (DVDs) that are rapidly-widespread optical information recording media having a higher recording density, red semiconductor lasers having a shorter wavelength in a 635 nm to 680 nm band are used to reduce the area of the optical spot. Guide grooves have different pitches depending on the type of optical information recording media. It is required that a single device performs recording and playback on two or more types of optical information recording media in different standards. In the case of DVDs, DVD-Rs having a guide groove pitch of 0.74 μm and DVD-RAMs having a guide groove pitch of 1.23 μm, for example, are used and one device needs to be employed for such DVDs. In view of this, a conventional optical pickup device as illustrated in FIG. 7 was proposed (see, for example, Japanese Unexamined Patent Publication No. 2004-145915).
FIG. 7 is a view schematically illustrating an example of a configuration of a conventional optical pickup device.
In FIG. 7, reference numeral 1 denotes a semiconductor laser light source, reference numeral 2 denotes a half mirror (or a beam splitter), reference numeral 3 denotes a collimator lens, reference numeral 4 denotes an object lens, reference numeral 5 denotes a detection lens, reference numeral 6 denotes an optical information recording medium, reference numeral 7 denotes a photodetector having photodetector surfaces partitioned according to a prescribed pattern, reference numeral 8 denotes a lens holder, reference numeral 9 denotes a two-dimensional actuator composed of electromagnetic circuits and reference numeral 10 denotes a diffraction grating.
As illustrated in FIG. 7, the object lens 4 is fixed in the lens holder 8 and driven by the two-dimensional actuator 9 in two directions: the direction along the optical axis (i.e., the direction of a laser beam 25 emitted from the semiconductor laser source 1) and the tracking direction. Hereinafter, the direction along the optical axis will be referred to as a focus direction. The diffraction grating 10 is placed between the semiconductor laser source 1 and the half mirror 2. The laser beam 25 emitted by the semiconductor laser source 1 is diffracted and separated by the diffraction grating 10 into at least three beams: a 0th order beam; a +1st order diffracted beam; and a −1st order diffracted beam (not shown). The separated beams are reflected by the half mirror 2 and then reach the object lens 4 via the collimator lens 3. The 0th order beam, the +1st order diffracted beam and the −1st order diffracted beam diffracted and separated by the diffraction grating 10 are caused to separately converge on the recording surface of the optical information recording medium 6 by the object lens 4 to form three convergence spots. At this time, as shown on the left side of FIG. 8, the three convergence spots 12 (corresponding to the 0th order beam), 13 (corresponding to the +1st order diffracted beam) and 14 (corresponding to the −1st order diffracted beam) are formed at a time on one of guide grooves 11 periodically (at a pitch TD) provided on the optical information recording medium 6. That is, the convergence spots 12, 13 and 14 are arranged substantially in a line.
In addition, as illustrated in FIG. 7, the beams reflected from the convergence spots on the optical information recording medium 6 travel reversely on almost the same optical path to the half mirror 2 through the object lens 4 and the collimator lens 3 in this order. Part of light quantity of the reflected beams passes through the half mirror 2 and is incident on photoreceptor surfaces of the multi-face photodetector 7 via the detection lens 5. Based on signals obtained by the photoreceptor surfaces of the photodetector 7, object lens position control signals such as a focus error signal and a tracking error signal and an information signal which has been recorded on the recording surface of the optical information recording medium 6 are detected by arithmetic circuitry. As shown on the right side of FIG. 8, the tracking error signal is detected by the processing of arithmetic circuitry having a similar configuration to that used in a conventional differential push-pull (DPP) method on signals detected by photoreceptor surfaces 7a (i.e., a photoreceptor surface for the reflected beam of the convergence spot 12), 7b (i.e., a photoreceptor surface for the reflected beam of the convergence spot 13) and 7c (i.e., a photoreceptor surface for the reflected beam of the convergence spot 14) of the photodetector 7. This arithmetic circuitry is composed of, for example, subtracters 15a, 15b and 15c connected to the respective photoreceptor surfaces 7a, 7b and 7c each divided into two, an adder 16 connected to the subtracters 15b and 15c, an amplifier 17 connected to the adder 16, and a subtracter 18 connected to the subtracter 15a and the amplifier 17.
FIG. 9 is a plan view illustrating a grating pattern of the diffraction grating 10 used in the conventional optical pickup device illustrated in FIG. 7. As illustrated in FIG. 9, while grooves 19a, 20a and 21a are formed at even intervals on the grating surface of the diffraction grating 10, the grating surface is partitioned into at least three areas 19, 20 and 21 by parting lines that are orthogonal to the grooves 19a, 20a and 21a. In other words, the grating surface is partitioned into at least three areas 19, 20 and 21 by parting lines parallel to the tracing direction (i.e., the tangential direction in FIG. 8) of the optical information recording medium 6. The grooves 19a, 20a and 21a are formed in the respective areas 19, 20 and 21. The area 20 at the center of the grating surface of the diffraction grating 10 has a given width W along the direction in which the grooves 20a extend. The phase of the grooves 19a periodically formed in the area 19 adjoining the central area 20 is ahead of that of the grooves 20a periodically formed in the central area 20 by 90° (i.e., is differentiated from that of the area 20 by +90°). That is, the interval of the grooves 19a in the area 19 is shifted by approximately one-fourth of that of the grooves 20a in the central area 20. Meanwhile, the phase of the grooves 21a periodically formed in the area 21 adjoining the central area 20 on the side opposite the area 19 is behind that of the grooves 20a in the central area 20 by 90° (i.e., is differentiated from that of the area 20 by −90°). That is, the interval of the grooves 21a in the area 21 is shifted by approximately one-fourth of the grooves 20a in the central area 20 in the direction opposite the direction of the shift of the grooves 19a in the area 19. Thus, the phase of the grooves 19a periodically formed in the area 19 is shifted from the phase of the grooves 21a periodically formed in the area 21 by 180°. That is, the arrangement of the grooves 19a in the area 19 is shifted from the arrangement of the grooves 21a in the area 21 by half a pitch.
The aforementioned conventional optical pickup device employing a tracking error detection method called an in-line DPP method enables stable tracking error detection on a plurality of optical information recording media having different guide groove pitches.